Regulated switching power supplies are often operated in parallel. However, when two or more regulated switching power supplies are so operated under usual conditions, less than all of the power supplies will carry the load, and the remaining power supplies will carry no load, unless the load is sufficiently high so that all of the power supplies are required to carry the load.
When the regulated switching-type power supplies are operated in parallel for redundancy purposes, for example, the load does not normally require all of the power supplies, so that the condition exists in that one or more of the power supplies is effectively shut off during normal operation. This is because the reason for the redundancy is to permit the remaining power supplies to carry the full load in the event that one of the power supplies should fail.
The reason why one or more of the paralleled power supplies is effectively shut off in such a redundancy system is because the voltage control regulating circuit in the individual power supply has virtually infinite gain for direct current. Accordingly, if the voltage setting of any one power supply falls below the voltage setting of the other power supplies, even by less than 1 millivolt, its voltage regulating circuit will cut the supply duty cycle of the particular power supply to zero.
As is well known, switching power supplies provide output pulses which are width modulated by their voltage regulating circuits to establish the output voltage of the power supplies. When the duty cycle of any individual power supply is reduced to zero, the output pulses from that power supply disappear.
However, the disappearance of the output pulses from any particular power supply may mean either that the particular power supply has failed or has no input power; or that the particular power supply is fully operational, but that its output voltage has been regulated to a value slightly below the voltage of the other paralleled power supplies so as to reduce its duty cycle to zero.
It is essential in the redundancy-type system of the type described above, that the operator be informed at all times that all of the power supplies are operational. Then, if any one of the individual power supplies has failed, it can be removed from the system and replaced.
Such knowledge is also important in parallel systems with forced current sharing in order that a failed power supply can be disconnected from the current sharing system before it drags down the common output.
"Keep alive" circuits have been proposed in the prior art in an attempt to determine whether any of the individual regulated switching power supplies in a paralleled system have failed. These prior art "keep alive" circuits attempt to override the voltage control regulating circuit in the individual power supplies by forcing a minimum duty cycle in each power supply. Accordingly, for each power supply, so long as the power pulses appear, the supply is operational. However, when no power pulses appear, the indication is that the particular power supply has failed.
The prior art "keep alive" circuit forces a minimum duty cycle on the pulse width modulator of the corresponding power supply even when the voltage regulator circuit indicates that the output voltage is too high. This creates a problem in that a substantial minimum load is required to prevent the power supply output from going extremely high. This is because the power switch must be turned on every cycle.
Another problem with the prior art "keep alive" circuit is that it is difficult to force a minimum duty cycle in the most commonly used integrated circuit pulse width modulators, such as the TL494.
The problems encountered by the prior art "keep alive" circuits are overcome in the system of the present invention by introducing a minimum pulse rate to the switching power supply power output pulses, rather than by attempting to force a minimum duty cycle.